Tennessee Onions

Cooking spray
2 ½ lb. sweet onions (about 3 large onions), sliced crosswise into ¼-in.-thick slices and separated into rings
1 tsp. dried thyme
1 tsp. dried parsley flakes
1 tsp. garlic salt
½ tsp. dried oregano
½ tsp. dry mustard
¼ tsp. cayenne pepper
¼ cup salted butter, cut into 1/4-in.-thick pieces
4 oz. mild Cheddar cheese, shredded (about 1 cup)
4 oz. smoked Gouda cheese, shredded (about 1 cup)

Preheat oven to 350°F. Coat a 13- x 9-inch baking dish with cooking spray.
Place onions in a large bowl. Sprinkle with thyme, parsley, garlic salt, oregano, mustard, and cayenne pepper; toss gently to coat.
Arrange evenly in prepared baking dish. Arrange butter evenly over onions, and sprinkle with Cheddar and Gouda.
Cover with aluminum foil, and bake in preheated oven until onions are soft and sides are bubbly, about 40 minutes.
Remove foil. Bake at 350°F until top is golden, about 30 minutes.